Method for exchanging authentication information between a communication entity and an operator server

ABSTRACT

An exchange of information between a communication entity e.g. a mobile telephone and the operator server upon which it is dependent, in order to carry out authentication operations by exchanging keys and using cryptography algorithms. According to the invention, an algorithm is used, comprising: algorithmic treatments ( 35 A,  35 B) using, as input data, all or part of a random number (FAND) and operating keys (KA, KA2-Kd) derived from two keys (K; Kd), and a combined treatment using data derived from said 10 algorithmic treatments in order to provide results (SRES, K.) enabling the linkage.

The invention relates to a method of exchanging authentication information between a communication entity, in particular a wireless mobile communication entity, and a corresponding operator server with which a call may be set up, possibly through the intermediary of a third party network. The invention applies in particular to exchanging information between a mobile telephone and its operator server, in particular to execute authentication operations by exchanging keys and using cryptography algorithms to verify the rights of said entity before setting up a call.

In the field of mobile telephones, a mobile telephone user must enter into communication with his operator server. To this end, radio communication is set up between the mobile entity (the mobile telephone) and the operator server, generally through the intermediary of a third party network (and the operator server thereof) whose telecommunication means are best able to receive the signal emitted by the mobile telephone, given its location at the time it seeks to set up the call.

Once the call has been set up, an authentication phase begins to verify the rights of the mobile entity. A telephone conversation is authorized if the cryptographic authentication process succeeds. For example, the GSM group has defined a function known as the A₃-A₈ function to implement the cryptographic process that enables the operator server concerned to authenticate a mobile telephone seeking to connect to it. This function is executed by a microcircuit card in the mobile telephone known as the subscriber identity module (SIM). In addition to authenticating the user, the A₃-A₈ function generates a temporary key K_(c) for securing subsequent communication between the telephone and the operator server by encrypting a portion of the traffic. The algorithms for implementing this function are specific to the operator. The input to this kind of algorithm comprises a random number of 16 bytes and a key of 16 bytes. The A₃ output is an authentication value SRES of 4 bytes and the A₈ output is a key K_(c) of 8 bytes. The GSM standard describes one example of an algorithm implementing the A₃-A₈ function known as the COMP 128-1 algorithm. However, it has been demonstrated that this algorithm is easy to break. The invention proposes another solution providing an algorithm that is more secure, offers improved performance, and is compatible with the GSM standard, i.e. that uses the same inputs and generates the same outputs. The invention is also aimed at an algorithm that is easy to implement, in particular on the operator server side.

The invention relates more particularly to a method of exchanging authentication information between an operator server and a communication entity, typically a mobile telephone, in which said entity and said server have the same first key and said server generates a random number and sends it to said entity for the parallel implementation of an algorithm in said entity and said server and for comparison of the results generated in said entity and said server in order to validate said authentication, which method is characterized in that said entity and said server comprise at least one common secret second key and said algorithm implemented in said entity and said server comprises:

-   -   algorithmic processes using as input data all or a portion of         said random number and operating keys derived from said first         and second keys, and     -   a combinatorial process using data derived from said algorithmic         processes to generate corresponding precited results.

In particular, said algorithm may comprise two similar algorithmic processes, cited above, each using two operating keys.

For example two respective portions of said random number, such as two separate portions of said number, may be used as input data for the two similar algorithmic processes referred to above. In particular, the less significant bytes of said random number may be used as input data for one of the two similar processes and the more significant bytes of the same random number may be used as input data for the other of the two similar processes.

A number of prior art algorithmic processes may be used, for example an “encryption function” algorithm, well known examples of which include the DES, RSA and AES algorithms, or a “key dependent one-way hashing function” algorithm, better known in the art as the message authentication code (MAC) algorithm. A hashing function of the above kind is described in the following books:

-   -   Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone,         “Handbook of Applied Cryptography”, 1997, pages 325-326; and     -   Bruce Schneier, “Crytopgraphie Appliqué” [“Applied         crytography”], 2nd edition, John Wiley & Sons, Inc., 1996, pages         479-480.

It is at present considered to be advantageous for each of said two similar algorithmic processes to execute a triple DES (3 DES) algorithm known in the art.

A triple DES algorithm using two keys is described in the following documents:

-   -   ANSI X9.17, American National Standard—Financial institution key         management (wholesale), ASC X9 Secretariat—American Bankers         Association, 1985;     -   ISO 8732, Banking—Key management (wholesale), International         Organization for Standardization, Geneva, Switzerland, 1987         (first edition, confirmed 1992); and     -   Alfred J. Menezes, Paul C. van Oorschot, Scott A. Vanstone,         “Handbook of Applied Cryptography”, 1997, page 272.

The secret second key referred to above may be called a diversification key in that it is specific to a particular operator server and to all the telephone entities attached thereto. Another operator server and all the corresponding mobile telephones could use the same type of algorithm with a different secret key or diversification key.

The invention will be better understood and other advantages of the invention will become more clearly apparent in the light of the following description of a mobile telephone network embodying the principle of the invention, which description is given by way of example and with reference to the appended drawings, in which:

FIG. 1 is a diagram depicting exchanges of information between a mobile telephone and an operator server via a third party network; and

FIG. 2 is a block diagram of an algorithmic processing cell implementing the A₃-A₈ function of the method according to the invention.

FIG. 1 shows the main entities involved in a process of exchange of information between a mobile telephone that subscribes to a mobile telephone network and an operator server of the network. The mobile telephone 11 comprises a short range transceiver and a microcircuit card 12 in which data and authentication algorithms are stored, among other things. The operator server 14 of the network to which the owner of the mobile telephone subscribes is generally situated at too great a distance for direct radio communication with the mobile telephone to be possible. For this reason communication is often established with a radio relay station 15 of a third party network connected to an operator server 17 capable of processing information and transmitting it to the operator server 14 of the network to which the mobile telephone belongs.

The microcircuit card includes, in particular, means 20 a for executing software capable of implementing the A₃-A₈ function, i.e. of generating the value SRES and a key K_(c). The operator server contains equivalent means 20 b. The microcircuit card further contains a key K_(i) specific to the owner of the mobile telephone. The operator server manages a database 23 containing a list of all subscribers and the corresponding keys K_(i).

At the time of a connection attempt, the mobile telephone 11 transmits in clear an international mobile subscriber identity (IMSI) number that is received by the third party network and forwarded by the latter to the operator server 14. The operator server uses this identification number to consult the database containing the list of subscribers and retrieve the key K_(i) stored in the memory of the microcircuit card 12. In response, the operator server generates a random number RAND and forwards it to the mobile telephone via the third party network.

From this moment, the mobile telephone 11 and the operator server 14 are in possession of the same input data, namely the random number RAND and the key K_(i), on the basis of which the same A₃-A₈ algorithm may be executed. The authentication algorithm is executed both in the microcircuit card of the mobile telephone and in an authentication unit of the operator server. If authentication succeeds, the results SRES and K_(c) are identical in the telephone and in the operator server. More particularly, the result SRES is an authentication word and K_(c) is a key that will be used as an encryption key for a function A_(s) for encrypting certain traffic data during the telephone call between the mobile telephone and the operator server. The authentication word SRES generated in the microcircuit card is sent to the operator server 17 of the third party. The authentication word SRES generated by the operator server 14 is sent to the operator server 17 of the third party network, in which the comparison is effected. Similarly, confidential traffic data is exchanged between the mobile telephone and the operator server 17 of the third party network, which receives the key K_(c) from the operator server 14.

This mode of operation is known in the art and conforms to the GSM standard.

FIG. 2 shows the means implemented in the card and in the authentication unit for executing the method according to the invention employing the algorithm implementing the A₃-A₈ functions defined by the standard. The random number RAND of 16 bytes is loaded into a register 30. A first key K_(i) of 16 bytes is loaded into a register 31. This key k_(i) is specific to the subscriber owning the mobile telephone 11 concerned and may also be retrieved from the database of the operator server 14.

A register memory 32 of 8 bytes contains a common secret second key K_(d) representative of the operator server 14. In other words, this second key is stored in each SIM card 12 of a user subscribing to the operator server. The latter naturally holds in its memory the same secret key K_(d). Another operator server belonging to another network could use the same algorithm but would have to use a different second key. Indeed the means employed comprise two similar algorithmic processing units 35 _(A), 35 _(B) each having an input E and an output S and receiving two operating keys derived from said first and second keys K_(i) and K_(d). More particularly, two separate portions (comprising the same number of bytes in the present example) of the random number RAND provide input data for the respective algorithmic processing units. In the present example, the less significant bytes of the random number RAND constitute input data for one of the two processing units (here the processing unit 35 _(A)) and the more significant bytes of the same random number constitute input data for the other of the two units (here the unit 35 _(B)). Similarly, in the present example, the less significant bytes of the key K_(i) are used as a first operating key K_(A1) of one of the two units and the more significant bytes of the same key K_(i) are used as the first operating key K_(A2) of the other of the two units. The two keys K_(A1) and K_(A2) have the same number of bytes and are two separate portions of the key K_(i). The second key K_(d) constitutes the second operating key of each of the two units 35 _(A), 35 _(B). The output of the unit 35 _(A) is connected to a register 37 of 8 bytes and the output of the other unit 35 _(B) is connected to a register 38 of 8 bytes. The intermediate results stored in the two registers 37, 38 constitute data for a combinatorial process that generates the results, namely the name SRES and the key K_(c). More specifically, in the present example, the four less significant bytes of the register 37 and the four more significant bytes of the register 38 are combined in an intermediate register 39 of 8 bytes to constitute a word of 8 bytes that forms the key K_(c). The key K_(c) is copied into an output register 40. The less significant bytes of the register 37 and the more significant bytes of the register 38 are combined by an exclusive-OR circuit 41 and the result is sent to an output register 42 of 4 bytes in which the authentication word SRES may be read.

In the present example, the two similar algorithmic processing units constitute a triple DES (data encryption standard) function that is known in the art. The DES encryption function is well known in the art and may be associated with its inverse function DES⁻¹. The triple DES function applies successively to the input data a DES algorithm governed by a first key K₁, a DES⁻¹ algorithm governed by a second key K₂, and a DES algorithm governed by the first key K₁. In the present example, the key K₁ is the operating key derived from the key K_(i) and the key K₂ is the diversification key K_(d).

As mentioned above, the triple DES function represented in FIG. 2 may be replaced by some other prior art encryption function or by a key-dependent one-way hashing function. This kind of algorithmic processing may be used by different operators with total security thanks to the introduction of the diversification key K_(d) specific to the operator. If another encryption function or a one-way hashing function is used, it is necessary to choose or to generate a function necessitating the use of two keys (K_(A1) or K_(A2) and K_(d)). If the function necessitates only one key, then the keys K_(i) and K_(d) may be combined. Using “triple DES” functions 35 _(A) or 35 _(B) is advantageous in that such units are routinely used in the art and available as such as standard subsystems.

Of course, the invention also relates to any microcircuit card usable in a communication entity comprising algorithmic processing and means for receiving operating keys derived from said keys K_(i), K_(d), one of the keys (K_(i)) being specific to the entity accommodating the microcircuit card 12 and the other key (K_(d)) being specific to the operator server 14 for the card enabling the use of said entity (the telephone 11). It also relates to an operator server comprising algorithmic processes analogous to those of this kind of card. In particular, the invention relates to any microcircuit card usable in a mobile telephone and comprising two similar algorithmic processing units 35 _(A), 35 _(B) each having an input E and an output S and means for receiving two operating keys derived from said first and second keys K_(i), K_(d). It finally relates to an operator server comprising two similar algorithmic processing units analogous to those of the above kind of card. 

1. Method of exchanging authentication information between an operator server and a communication entity, typically a mobile telephone, in which said entity and said server have the same first key (K_(i)) and said server generates a random number (RAND) and sends it to said entity for the parallel implementation of an algorithm in said entity and said server and for comparison of the results generated in said entity and said server in order to validate said authentication, characterized in that said entity and said server comprising at least one common secret second key (K_(d)) and said algorithm implementation in said entity and said server comprises: algorithmic processes (35A and 35B) using as input data all or a portion of said random number and operating keys derived from said first and second keys, and a combinatorial process using data derived from said algorithmic processes to generate corresponding precited results.
 2. Method according to claim 1, characterized in that said algorithm comprises two similar algorithmic processes (3 DES) each using two operating keys (K_(A1), K_(A2)-K_(d)).
 3. Method according to claim 1 characterized in that two respective portions of said random number (RAND), are used as input data of said two similar algorithmic processes.
 4. Method according to claim 3, characterized in that the less significant bytes of said random number are used as input data for one (35 _(A)) of said two similar processes and the more significant bytes of the same random number are used as input data for the other (35 _(B)) of said two similar processes.
 5. Method according to claim 2, characterized in that each of said two similar algorithmic processes consists in executing an encryption function algorithm known in the art.
 6. Method according to claim 2, characterized in that each of said two similar algorithmic processes consists in executing a 3 DES algorithm known in the art.
 7. Method according to claim 2, characterized in that each of said two similar algorithmic processes consists in executing a key-dependent one-way hashing function known in the art.
 8. Method according to claim 2, characterized in that it consists in combining at least one portion of the data obtained from each of said algorithmic processes through an exclusive-OR function (41) to form an authentication word (SRES).
 9. Method according to claim 2, characterized in that it consists in reconstituting a common encryption key (K_(c)) from portions of data obtained from said algorithmic processes.
 10. Method according to claim 2, characterized in that two respective portions of said first key (K_(i)) are used first operating keys for said two similar algorithmic processes.
 11. Method according to claim 9, characterized in that the less significant bytes of said first key are used as the first operating key of one (35 _(A)) of the two similar processes and the more significant bytes of said first key are used as the first operating key of the other (35 _(B)) of the two similar processes.
 12. Method according to claim 2, characterized in that two respective portions of said random number (RAND), are used as input data of said two similar algorithmic processes. 